CORDIS

Energy efficiency is the most important issue for our time. Oil will continue getting more expensive, global warming requires us to reduce our carbon footprint, plastic will become more scarce, and as a consequence, looking at the live music industry, one of the most inefficient parts in terms of transportation, weight, size is the PA system (especially loud ones).

Design brief: Your mission...Make a 1.6kW rig (in stereo) which can fit on the back seat of a hatchback (sub-compact for transatlantic readers) with a bass response down to 27Hz (flat) and it has to be portable by one person alone and be made of sustainable materials as far as possible. Now that would be nice, wouldn’t it? Here’s one I made earlier (ably demonstrated by my trusty assistant Ben Vickers):

Cabinets Of Relative Dimensions In Space

Here is Ben with one of the beasties. Looks like a hat-box, Ben!

The fundamental problem, if you want lots of bass (in a reflex cabinet), is how to get the volume of air. Imagine holding a piece of elastic with a weight on the end of it. As you move your hand up and down the weight moves too but not in sync with your hand. At some rate (frequency) you will find that the weight is going down when your hand is going up. That’s what you’re looking for in a reflex cabinet. The speaker is like your hand (putting in the energy), the ports (normally tubes in the same face as the speaker) are the weight (resistance) and the volume of air is the elastic (because air is elastic, if you take a bicycle pump and put your finger over the end, you can squash the air). So what happens is you get twice the air coming out for the same movement going in.

Right then what’s needed is lots of air, and I calculated that if you want to get right down to 30Hz (or thereabouts), to flap the bottom of your trouser legs, you would need more than 10 cubic feet of it, so that back seat of a hatchback looks like a non-starter, especially in stereo (>20 cubic feet).

But, and this is the point, you only need a volume of air, that’s all. So you can make the volume of air any way you like bearing in mind; It is going to get a lot of force (with all that power) applied to it so it’s got to be strong; and of course the mission is to make it possible for one person to move it about.

Notice this thing is round? That’s what makes it so strong but as a side benefit you can roll it about.

Ben’s fingers are inside some slots - they are the ports, normally tubes, but again the ports are simply volumes of air acting as restrictors & they make great handles, too!

Right, Ben, let’s get on with it. First we are going to need to put some feet on this thing as that big bass speaker (2,400W peak 18“) points downwards. Let’s see it on its feet.

And he’s already taken the strap off (stops the various sections falling out when he rolls it around). The handle’s on the top surface: go on Ben, pull it up.

Oh, another speaker, and it’s a coaxial one, where the high frequency driver is fastened to the back and the sound squirts out of a hole in the magnet of the mid range driver, using the flare of the speaker cone as the horn.

Obviously there’s going to be a big empty space inside: let’s have a look Ben.

Anyone wondering how that top box fits? It would appear that the magnet on the bass driver interferes with the top box.

Not only a recess where the magnet was but a speakon connector too.

Tubes:

Notice the flange on the top, well that’s going to be turned upside down so it’s the bottom, same as the other tube, on top of each other.

Like this. Then the next one.